L-type calcium channels and μ-opioid receptors are involved in mediating the anti-inflammatory effects of naloxone

Background: We sought to elucidate the effects of naloxone on regulating the up-regulation of inflammatory molecules and activation of the transcription factor nuclear factor-kappaB (NF-κB) induced by endotoxin. Possible roles of the μ-opioid receptors and L-type calcium channels in mediating the effects of naloxone in this regard were also investigated. Materials and Methods: RAW264.7 cells were treated with phosphate buffered saline, naloxone, lipopolysaccharide (LPS), LPS plus naloxone, LPS plus naloxone plus morphine (i.e., the nonselective opioid receptors agonist), LPS plus naloxone plus fentanyl (i.e., the μ-opioid receptors agonist), or LPS plus naloxone plus BAY-K8644 (i.e., the L-type calcium channel activator). After harvesting, production of inflammatory molecules and expression NF-κB were evaluated. Results: The effects of LPS on inducing the up-regulation of macrophage inflammatory protein-2, tumor necrosis factor-alpha (TNF-α), interleukin (IL)-1β, IL-6, nitric oxide/inducible nitric oxide synthase, and prostaglandin E ₂/cyclooxygenase 2 were inhibited by naloxone. Naloxone also inhibited the effects of LPS on inducing NF-κB activation, including inhibitor-κB (I-κB) degradation, NF-κB nuclear translocation, and NF-κB-DNA binding. The effects of naloxone on inhibiting IL-1β up-regulation and NF-κB activation were enhanced by morphine. In contrast, the effects of naloxone on inhibiting IL-1β up-regulation and I-κB degradation were counteracted by fentanyl. Moreover, except for TNF-α, the effects of naloxone on inhibiting inflammatory molecules up-regulation and NF-κB activation were significantly counteracted by BAY-K8644. Conclusions: Naloxone significantly inhibited endotoxin-induced up-regulation of inflammatory molecules and NF-κB activation. The mechanisms may involve antagonizing the L-type calcium channels and, to a lesser extent, the μ-opioid receptors.